The present invention relates to a method and system for controlling frequency in general and to a method and system for providing an appropriate frequency, within predetermined error limits.
In the art, there are known methods and systems for controlling and moderating frequency. A conventional transceiver includes therein a local oscillator which, for example in radio frequency (RF) systems, is usually a frequency generating crystal. When the transceiver receives a signal in a given frequency fS, it has to shift the frequency fS of the signal to a desired frequency fd by producing an internal frequency fLO, using the local oscillator. Conventional frequency crystals are known to include a frequency shift which needs to be corrected, according to the received signal frequency fS.
Reference is now made to FIGS. 1 and 2. FIG. 1 is a schematic illustration of a prior art system, generally referenced 1, for controlling the internal frequency of a transceiver. FIG. 2 is a graphic illustration of the frequency versus voltage curve, referenced of a typical voltage-controlled-oscillator (VCO).
System 1 includes a voltage-controlled-oscillator 2 (VCO), an input frequency shift unit 5 and an output frequency shift unit 7, both connected to VCO 2, and a frequency estimator 4, connected to VCO 2 and to input frequency shift unit 5, for determining the required frequency correction.
Conventionally, a demodulator 6 is connected after the input frequency shift unit 5 and a modulator 8 is connected before the output frequency shift unit 7.
A received signal provided to the input frequency unit 5, incorporates a frequency fS. The VCO 2 generates an internal frequency fIN and provides it to the input frequency shift unit 5. The input frequency shift unit 5 produces a new signal having an intermediate frequency of fI, wherein fI=fSxe2x88x92fIN, and provides the new signal to the frequency estimating unit 4 as well as to the demodulator 6. It will be appreciated that conventional demodulators define a maximal allowed frequency difference xcex94fALLOWED between fI and fd, so that |fIxe2x88x92fd|xe2x89xa6xcex94fALLOWED. A conventional oscillator oscillates at a frequency which changes due to poor manufacturing quality, temperature changes and the like so that a situation where |fIxe2x88x92fd| greater than xcex94fALLOWED often occurs.
The frequency estimator 4 detects the signal provided by the frequency shift unit 5. If |fIxe2x88x92fd| greater than xcex94fALLOWED then fI has to be adjusted to equal fd. The frequency estimator 4 determines the frequency correction xcex94f=fdxe2x88x92fI, determines a respective voltage V(xcex94f) and provides the voltage V(xcex94f) to the VCO 2. Accordingly, the VCO 2 generates a corrected internal frequency fIN.
It will be noted that, as long as |fIxe2x88x92fd| greater than xcex94fALLOWED, the demodulator cannot properly process the incoming signal.
The modulator 8 produces a signal for transmitting at a frequency of fT and provides the signal to the output frequency shift unit. 7. The VCO 2 provides an internal transmitting frequency fOUT to the output frequency shift unit 7. In turn, the output frequency shift unit 7 produces a transmittable signal bearing a frequency of fOUT+fT which is the transmitted signal b broadcasting frequency.
It is noted that fIN and FOUT are generated around a main frequency fLO, generated by the local oscillator of the VCO unit 2.
Referring now to FIG. 2, when provided with a voltage at a value of v1, the VCO 2 is expected to provide the respective frequency f. A conventional VCO, having an internal inaccuracy, may provide, for a given voltage value of v1, a frequency in the range between fMIN and fmax. When maximum(fMAXxe2x88x92f,fxe2x88x92fMIN) greater than xcex94fALLOWED, the frequency correction cannot be deemed final.
Conventionally, after several cycles of detection and correction, the intermediate frequency fI should differ from the desired frequency fd by no more than xcex94fALLOWED. Until then, the demodulator 6 cannot perform properly.
It will be appreciated that the modulator 8 is also dependent on the accuracy of the frequency provided thereto by the VCO 2.
It is an object of the present invention to provide a novel method and a novel system for providing an appropriate frequency, within predetermined error limits.
According to the present invention there is thus provided a system for receiving a signal in a received frequency and shifting the received frequency to become a desired frequency, the system including a controllable oscillator for generating a first internal frequency, a frequency estimating unit connected to the controllable oscillator, a first frequency shift unit, connected to the controllable oscillator and to the frequency estimating unit, for shifting the received frequency according to the first internal frequency, thereby obtaining an initially shifted frequency and a second frequency shift unit connected to the first frequency shift unit and the frequency estimating unit for shifting the initially shifted frequency.
The frequency estimating unit determines a total frequency shift value from the desired frequency, the received frequency and the first internal frequency and it also determines a first frequency shift value and a second frequency shift value from the total frequency shift value.
Furthermore, the frequency estimating unit provides a first frequency change command to the controllable oscillator, so as to correct the first internal frequency according to the first frequency shift value, a second frequency change command to the second frequency shift unit, so as to shift the initially shifted frequency according to the second frequency shift value.
The frequency estimating unit may provides the first frequency change command to the controllable oscillator as an electrical voltage, with respect to the controllable oscillator characteristics.
The second frequency shift unit can be a hardware unit or a software unit. The second frequency shift unit may include means for performing a frequency shift by multiplying the initially shifted frequency by ej2xcfx80xcex94f, wherein xcex94f is the second frequency shift value.
The controllable oscillator may generates a second internal frequency, for example, for use in a combined transmitter receiver implementations a. In such a case, the system further includes a first output frequency shift unit, connected to the controllable oscillator for shifting a transmittable signal according to the second internal frequency.
For providing enhanced frequency correction according to the invention, the system further includes a second output frequency shift unit connected before the first output frequency shift unit and to the frequency estimating unit. The frequency estimating unit provides a command to the second output frequency shift unit to shift the transmittable signal according to the second frequency shift value thereby producing an initially shifted transmittable signal. Then, the first output frequency shift unit shifts the initially shifted transmittable signal according to the second internal frequency.
According to another aspect of the present invention there is provided a method for operating a system generating a controllable internal frequency fIN, a method for shifting a signal from an initial frequency fS towards a desired frequency fD to a final frequency fF, wherein |fDxe2x88x92fF| is less than a predetermined allowed frequency error fALLOWED.
The method includes the steps of:
determining a total frequency shift value FTOTAL according to the internal frequency fIN and the initial frequency value fS;
determining a first frequency shift value xcex94f1 from the total frequency shift value fTOTAL and the predetermined allowed frequency error fALLOWED;
determining a second frequency shift value xcex94f2 from the total frequency shift value fTOTAL and the first frequency shift value xcex94f1;
adjusting the internal frequency fIN according to the first frequency shift value xcex94f1 thereby shifting the initial frequency fS so as to produce a shifted initial frequency fSHIFTED; and
shifting the shifted initial frequency fSHIFTED according to the second frequency shift value xcex94f2 thereby producing the final frequency fF.
The sum of the first frequency shift value xcex94f1 and the second frequency shift value xcex94f2 equals the total frequency shift value fTOTAL. The first frequency shift value xcex94f1 defines a frequency error fERROR which is less than the predetermined allowed frequency error fALLOWED. Since xcex94f2 includes an insignificant frequency error, thus, according to the invention, it is guarantied that the total frequency correction equals frequency error fERROR.